Active Noise Control (ANC) systems attenuate undesired noise using feedforward and feedback structures to adaptively remove undesired noise within a listening environment, such as within a vehicle cabin. ANC systems generally cancel or reduce unwanted noise by generating cancellation sound waves to destructively interfere with the unwanted audible noise. Destructive interference results when noise and “anti-noise,” which is largely identical in magnitude but opposite in phase to the noise, reduce the sound pressure level (SPL) at a location. In a vehicle cabin listening environment, potential sources of undesired noise come from the engine, the interaction between the vehicle's tires and a road surface on which the vehicle is traveling, and/or sound radiated by the vibration of other parts of the vehicle. Therefore, unwanted noise varies with the speed, road conditions, and operating states of the vehicle.
A Road Noise Cancellation (RNC) system is a specific ANC system implemented on a vehicle in order to minimize undesirable road noise inside the vehicle cabin. RNC systems use vibration sensors to sense road induced vibrations generated from the tire and road interface that leads to unwanted audible road noise. This unwanted road noise inside the cabin is then cancelled, or reduced in level, by using speakers to generate sound waves that are ideally opposite in phase and identical in magnitude to the noise to be reduced at one or more listeners' ears. Cancelling such road noise results in a more pleasurable ride for vehicle passengers, and it enables vehicle manufacturers to use lightweight materials, thereby decreasing energy consumption and reducing emissions.
An Engine Order Cancellation (EOC) system is a specific ANC system implemented on a vehicle in order to minimize undesirable engine noise inside the vehicle cabin. EOC systems use a non-acoustic signal, such as a revolutions-per-minute (RPM) sensor, to generate a signal representative of the engine speed as a reference. This reference signal is used to generate sound waves that are opposite in phase to the engine noise audible in the vehicle interior. Because EOC systems use a signal from an RPM sensor, they do not require vibrations sensors.
RNC systems are typically designed to cancel broadband signals, while EOC systems are designed and optimized to cancel narrowband signals, such as individual engine orders. ANC systems within a vehicle may provide both RNC and EOC technologies. Such vehicle-based ANC systems are typically Least Mean Square (LMS) adaptive feed-forward systems that continuously adapt W-filters based on noise inputs (e.g., acceleration inputs from the vibrations sensors in an RNC system) and signals of error microphones located in various positions inside the vehicle's cabin. A feature of LMS-based feed-forward ANC systems and corresponding algorithms is the storage of the impulse response, or secondary path, between each error microphone and each anti-noise speaker in the system. The secondary path is the transfer function between an anti-noise generating speaker and an error microphone, essentially characterizing how an electrical anti-noise signal becomes sound that is radiated from the speaker, travels through a vehicle cabin to an error microphone, and becomes the microphone output signal.
ANC systems employ modeled transfer characteristics, which estimate the various secondary paths, in the adapting the W-filters. Noise cancellation performance degradation, noise gain, or actual instability can result if the modeled transfer characteristic of the secondary path stored in the ANC system differs from the actual secondary path within the vehicle. The actual secondary path may deviate from the stored secondary path model, typically measured on a “golden system” by trained engineers, when a vehicle becomes substantially different from the reference vehicle or system in terms of geometry, passenger count, luggage loading, or the like. Other differences could include or speaker or microphone unit-to-unit variation, aging, or failure, non-identical speaker replacement or wiring errors.